The invention relates generally to methods and apparatus for CT imaging, and more particularly to methods and apparatus for performing computed tomography (CT) imaging utilizing first and second reconstructions of at least partially common attenuation measurements and EKG gating information.
In certain known computed tomography (CT) imaging systems, an x-ray source transmits x-ray beams through an object of interest. The x-ray beams pass through the object being imaged, such as a patient. The beams, after being attenuated by the object, impinge upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. Attenuation measurements from the detectors are acquired separately for each detector element and collectively define a projection data set or transmission profile.
The x-ray source and the detector array are rotated on a gantry within an imaging plane around the object to be imaged such that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, e.g., projection data set, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. The projection data sets are processed to construct images that correspond to two-dimensional slices taken through the object at various angles. One exemplary method for forming an image from a projection data set is referred to as filtered back projection technique.
The medical community has become increasingly concerned with reducing the x-ray dose to a patient during a CT examination. In Europe, for example, strict guidelines have been instituted to prevent potential danger of exposing patients and operators to excessive x-ray radiation. New uses of CT imaging have increased the interest in limiting x-ray exposure of patients. In cardiac screening CT, for example, asymptomatic patients undergo routine CT scans to detect calcification in coronary arteries. However, in certain instances, to provide cardiac screening, conventional CT systems and reconstruction techniques may perform more scans of the patient than performed by scans of non-cardiac anatomy. For example, additional scans may be taken to obtain more projection data sets to accurately reconstruct the heart. The additional scans are used to obtain sufficient information at various points in the cardiac cycle to support a reconstruction that is compensate for motion artifacts. Also, even if the radiation within an individual cardiac examination remains the same as in noncardiac examinations, as CT examinations are used for more types of screening, the patient will receive more radiation (e.g., a virtual colonoscopy, upper and lower GI examinations, vascular examinations and the like).
Further, the size of detectors used in CT systems continues to increase in order to obtain information regarding larger and larger regions of interest. In the example of cardiac screening, the region surrounding the heart (e.g. the lungs and other anatomy) is also of interest as well as the heart itself. CT systems with large detectors, when used in more frequent scans or longer scans, potentially increase the overall dosage to the patient.
It is desirable to provide a more efficient CT system and reconstruction techniques that reconstruct moving objects, such as the heart and reconstruct large field of views.
Also, certain conventional CT systems actively manage the tube current utilized to control the x-ray source. For example, conventional CT systems may modulate the tube current based on the view angle, where the current is increased when the x-ray source and detector are located laterally on opposites sides of a patient. The tube current may then be decreased as the x-ray source and detector rotate to locations above and below the patient. Another form of tube current modulation is ECG gated modulation, where the tube current is increased and decreased based on the cardiac cycle. In ECG gated modulation, the tube current may be increased to a peak level at the diastole point in the cardiac cycle and may be decreased to a minimum level at the systole point in the cardiac cycle. A third form of tube current modulation is Z-profile modulation, where the tube current is modulated in the Z-direction based on a Z-axis attenuation profile. The Z-axis attenuation profile may be predetermined based on general patient population statistics. The Z-axis attenuation profile may be determined during a scout scan of a patient. During the scout scan, the patient is scanned with a very low dose and attenuation measurements are obtained along the length of the patient. The attenuation measurements are used to form an attenuation profile of the patient in the Z-direction.
It is desirable to provide improved tube current modulation techniques that better manage the application of x-ray energy relative to particular organs of interest. It is also desirable to combine the use of multiple reconstruction techniques, each technique of which exhibits certain advantages depending upon characteristics of the scan data (e.g., noise, scan data overlap, gating information, etc.).